“Prefers reduced motion” is a setting people can turn on in their device to ask apps and websites to use less movement. A website can check for this preference and tone down or remove animations that are not needed.
This matters because large or frequent movement can make some people feel dizzy or uncomfortable. It is like offering a quieter version of an effect, while keeping the page usable. The setting does not disable every animation by itself; the website's designers need to respect it and provide a calmer experience.